Check isDiscardableIfUnused, rather than hasLocalLinkage, when bumping
GlobalValue linkage up to ExternalLinkage in the ExtractGV pass. This
prevents linkonce and linkonce_odr symbols from being DCE'd.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176459 91177308-0d34-0410-b5e6-96231b3b80d8